1. Field of the Invention
The present invention relates to a mass analyzer for classifying a substance according to a mass thereof, and in particular, to control of ejection of ions from an ion source of the mass analyzer.
2. Description of the Related Art
A mass analyzer classifying a substance depending on a mass thereof has been used in apparatuses such as a mass spectrometer analyzing components of a sample in the qualitative and quantitative fashions, an ion implant apparatus implanting predetermined ion species, and a secondary ion mass spectrometer (SIMS) analyzing secondary ions.
For the classification of substances in an ion beam by the mass of each substance, there have been known a method using a combination of an electric field and a magnetic field and a method using a quadrupole.
In order to generate an ion beam, ions are produced in an ionization chamber associated with an ion source and a voltage having a predetermined polarity with respect to an inner wall (including an emission slit) of the ionization chamber so as to repel the ions is applied to a repeller electrode in the ionization chamber, thereby outputting an ion beam for the emission slit of the ionization chamber. When an acceleration voltage is applied across the ionization chamber and an electric lens, the electric beam is accelerated so as to be passed through the lens. The accelerated ion beam is shaped in the lens system to obtain an ion beam having a predetermined shape. Thereafter, using a quadrupole or a combination of an electric field and a magnetic field, the ion beam is dispersed and then only desired ion species are converged depending on the mass. Incident ions are detected by a detector to measure the amount of ions, which is then analyzed by a data processing system, thereby effecting the qualitative and quantitative analyses of the sample. In a case of an ion implantation, ions are implanted into a target. Also in this case, the amount of ions must be measured. For the quantitative accuracy, it is desired to generate an ion beam which includes a fixed amount of ions and preferably contains the maximum available amount thereof in any cases. To this end, it is a common practice to adjust a potential of the repeller electrode with respect to the potential of the ionization chamber. Incidentally, an acceleration voltage having a voltage of about .+-.4 kV with respect to the ground potential is applied to the ionization chamber. Namely, the voltage of the repeller electrode is at most .+-.10 V with respect to the voltage of the ionization chamber (i.e. the reference voltage) and is higher than the ground voltage by about .+-.4 kV. Consequently, care must be exercised for the high-voltage operation.
The JP(UM)-A-No. 53-24685 has disclosed a variable resistor for controlling a repeller voltage onto which such an acceleration voltage is imposed. A mechanical rotary shaft of the variable resistor is extended by means of an extension member made of an insulating material so as to be linked with a dial disposed in an operator's panel, thereby increasing a distance between the operator's panel and the variable resistor to which a high voltage is applied.